Multivibrator circuit

ABSTRACT

High speed multivibrator circuits operable with lower operating voltages are disclosed. A multivibrator circuit comprises an operating voltage source, first and second nonlinear amplifier components, each comprising a first and a second main electrode and a control electrode, wherein the first main electrode of the second amplifier component is connected to control the control electrode of the first amplifier component, and the first main electrode of the first amplifier component is connected to control the control electrode of the second amplifier component. A capacitor is connected between the second main electrode of the first and second amplifier components. First and second resistors connect the first main electrode of the first and second amplifier components to a first potential of the operating voltage source. A pull-down circuit is connected between the second main electrodes of the first and second amplifier components and a second potential of the operating voltage source. The pull-down circuit comprises third and fourth amplifier components, acting as active pull-down components which are cross-connected such that they are alternately in conducting and nonconducting states due to forced control by the states of the first and second amplifier components.

FIELD OF THE INVENTION

The invention relates generally to multivibrators and specifically toso-called emitter-coupled multivibrators.

BACKGROUND OF THE INVENTION

Current- and voltage-controlled oscillators (ICO and VCO) are importantcomponents in the structures of transmitters and receivers. Whenapplications to portable wireless communications systems are concerned,the main requirements for VCO/ICOs are: an operational frequency rangeof 1 to 20 GHz, a very low phase noise and the lowest possible operatingvoltage and power consumption. Depending on the structure, acommunications device may comprise several VCO/ICOs needed for differentpurposes, e.g. frequency conversion, synthetization, modulation, etc.Their performance affects strongly the performance of the wholecommunications unit. On the other hand, the demand to implement theseoscillators for silicon technologies faces several problems.

Oscillator circuits, i.e. oscillators, can be implemented by manydifferent circuit structures. One of them is an astable (free-running)multivibrator. FIG. 1 shows a conventional emitter-coupled multivibratorcircuit, which has been used for implementing voltage-controlledoscillators (VCO). The circuit comprises two transistors Q1 and Q2,between which is provided a positive feedback by cross-coupling eachtransistor base to the collector of the other transistor. The positivefeedback and series resonance circuits Rc1-C and Rc2-C constituted bythe resistors RC1 and RC2 and a capacitance C lead to that the output ofthe multivibrator oscillates continuously between two states, after theoscillation once has been trigged. The oscillation frequency isdetermined by the component values of the RC series resonance circuits.The oscillation frequency can be controlled by changing some of thesecomponent values, typically the capacitance C.

The speed of such a multivibrator circuit (maximum resonance frequency)depends primarily on the properties of the transistors Q1 and Q2. Oneknown way of increasing the speed of the multivibrator circuit is toprovide a positive feedback from the collector of one transistor to thebase of the other transistor via a buffer transistor. This enables ahigher base current, which in turn discharges parasitic capacitances ofthe base circuit of the transistor faster and accelerates thus theswitching of the transistor from one state to another.

The lowest possible operating voltage Vcc is at least 1.1V, when it isassumed that current sources 3 and 4 are ideal, i.e. no voltage loss isprovided in them. When the ideal current sources are replaced by somepractical circuit structure, such as current mirrors constituted by MOStransistors M11 M21 and M31 in FIG. 2, Vcc increases. The currentsources M11, M21 and M31 of FIG. 2 need a direct voltage of about 0.7 Vacross them, the total Vcc being at least about 1.8 V. The volumeresistance of the MOS transistors is the main reason for a high drainsource voltage Vds, when the transistors are on. Returning to theoperating principle of the circuit, it can be stated that current pathsare either Q1-C-M21 or Q2-C-M11 and that the current mirrors produce astable current through the reference capacitor C, which is the mainreason for the typical low phase noise. In search of a new way ofincreasing the speed, the reference capacitor cannot be decreased muchmore, because it will be of the order of the parasitic capacitances,which leads to the fact that a controlled planning of the circuit is notpossible.

Nowadays there is, however, a need of ever-increasing speeds while anoperating voltage as low as possible is desired, especially inelectronic equipments using battery power supplies.

SUMMARY OF THE INVENTION

An object of the present invention is a novel multivibrator circuithaving a higher speed th an the circuits according to the prior art.

Another object of the present invention is a novel multivibrator circuithaving a lower operating voltage than the circuits according to theprior art.

The invention relates to a multivibrator circuit, comprising

an operating voltage source,

a first non-linear amplifier component comprising a first and a secondmain electrode and a control electrode,

a second non-linear amplifier component comprising a first and a secondmain electrode and a control electrode, the first main electrode of thesecond amplifier component being connected to control the controlelectrode of the first amplifier component, and respectively, the firstmain electrode of the first amplifier component being connected tocontrol the control electrode of the second amplifier component,

a capacitive component connected between the second main electrode ofthe first amplifier component and the second main electrode of thesecond amplifier component,

a first and a second resistor, via which the first main electrode of thefirst amplifier component and the first main electrode of the secondamplifier component, respectively, are connected to a first potential ofthe operating voltage source. The oscillator is characterized in that itadditionally comprises a pull-down circuit between the second mainelectrodes of the first and the second amplifier component and a secondpotential of the operating voltage source.

The invention also relates to a multivibrator circuit, which ischaracterized in that it comprises

a third amplifier component connected in series between the second mainelectrode of the first amplifier component and the second potential ofthe operating voltage source,

a fourth amplifier component connected in series between the second mainelectrode of the second amplifier component and the second potential ofthe operating voltage source,

a fifth amplifier component, in which the first main electrode isconnected to the first potential of the operating voltage source, thesecond main electrode is operationally connected to the second potentialof the operating voltage source via a resistor or a current source andthe control electrode is connected to the second main electrode or thecontrol electrode of the first amplifier component,

a sixth amplifier component, in which the first main electrode isconnected to the first potential of the operating voltage source, thesecond main electrode is operationally connected to the second potentialof the operating voltage source via a resistor or a current source andthe control electrode is connected to the second main electrode or thecontrol electrode of the second amplifier component,

the control electrodes of the third and the fourth amplifier componentbeing cross-connected to the second main electrode of the fifth and thesixth amplifier component, respectively.

The multivibrator according to the invention is provided with the thirdand the fourth amplifier component, acting as active pull-downcomponents. The pull-down amplifier components are cross-connected insuch a way that they are alternately in conducting and non-conductingstate due to a forced control by the state of the first and the secondamplifier component. When the second amplifier component is innon-conducting state and the first amplifier component is in conductingstate, a third pull-down amplifier component connected between thesecond main electrode of the first amplifier component and the secondoperating voltage potential is in non-conducting state. A fourthpull-down amplifier component connected between the second mainelectrode of the second amplifier component and the second operatingvoltage potential is in conducting state pulling down the second mainelectrode to the second operating voltage potential. Then the circuithas only one circuit path via the first amplifier component, thecapacitive component and the fourth amplifier component.Correspondingly, when the first amplifier component is in non-conductingstate and the second amplifier component is in conducting state, thefourth pull-down component is in non-conducting state and the thirdpull-down amplifier component in conducting state. Then the circuit hasonly one circuit path via the second amplifier component, the capacitivecomponent and the third pull-down amplifier component. In this"double-cross-connected" multivibrator circuit, an output signalamplitude twice as high as in the prior art multivibrator circuits isachieved by pull-down technique at the same operating voltage.

The pulse shape of the multivibrator circuit according to the inventioncan be improved and its speed can be increased by applying controlsignals from the second main electrode or the control electrode of thefirst and the second amplifier component to the control electrodes ofthe fourth and the third pull-down amplifier component, respectively,via buffer amplifier components.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described with reference to theattached drawings, in which

FIGS. 1 and 2 are circuit diagrams, showing two emitter-coupledmultivibrator circuits according to the prior art,

FIGS. 3, 4, 5, 6 and 7 are circuit diagrams, showing differentdouble-cross-connected multivibrator circuits according to theinvention.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention is applicable to lowering operating voltage andincreasing speed in so-called emitter-coupled multivibrator circuits.Although the multivibrator circuits according to the prior art, shown inFIGS. 1 and 2, as well as the multivibrator circuits according to theinvention, shown in FIGS. 3 to 7, use bipolar transistors as amplifiermeans, the circuit solutions according to the invention may use any typeof non-linear amplifier components, in principle, such as MOS, CMOS,SOI, HEMT and HBT transistors, microwave tubes and vacuum tubes. Thenames of the electrodes may vary in these components. The mainelectrodes of a bipolar transistor are a collector and an emitter andthe control electrode is a base. In FETs, the corresponding electrodesare a drain, a source and a gate. In vacuum tubes, these electrodes areusually called an anode, a cathode and a gate. Thus the termemitter-coupled multivibrator shall also be understood in thisconnection as a more general concept, covering e.g. the termscathode-coupled or source-coupled multivibrator.

FIG. 3 shows an emitter-coupled multivibrator circuit according to afirst embodiment of the invention. The circuit comprises four NPNbipolar transistors Q1, Q2, Q3 and Q4. A collector electrode of thetransistor Q1 is connected via a resistor Rc1 to a positive operatingvoltage Vcc of an operating voltage source 1 and an emitter is connectedto a collector of the transistor Q3. An emitter of the transistor Q3 isconnected to a lower operating voltage potential, e.g. 0V, of thevoltage source 1. A collector of the transistor Q2 is connected via aresistor Rc2 to the operating voltage Vcc and an emitter to a collectorof the transistor Q4. An emitter of the transistor Q4 is connected tothe operating voltage 0V. A capacitor C is connected between theemitters of the transistors Q1 and Q2. A positive feedback is providedbetween the transistors Q1 and Q2 by cross-connecting a base of Q1 tothe collector of Q2 and a base of Q2 to the collector of Q1.Correspondingly, a positive feedback is provided between the transistorsQ3 and Q4 by cross-connecting a base of Q3 to the collector of Q4(emitter of Q2) and a base of Q4 to the collector of Q3 (emitter of Q1).The positive feedbacks and series resonance circuits Rc1-C and Rc2-Cconstituted by the resistors Rc1, Rc2 and the capacitor C provide that amultivibrator output (e.g. emitter of Q2) oscillates between two states,when the oscillation once has been triggered. The resonance frequency ofthe circuit is set by the values of the components Rc1, Rc2 and C. Theresonance frequency can be controlled e.g. by controlling the values ofthese components in a manner known per se.

In the multivibrator circuit of the invention, the pull-down transistorsQ3 and Q4 replace the current sources of the conventional multivibratorcircuit shown in the FIGS. 1 and 2. Due to the cross-connection of thetransistors Q3 and Q4, they are alternately on and off, under forcedcontrol by the states of the transistors Q1 and Q2. Let us assume forinstance that the transistor Q1 is on and the transistor Q2 is off. Thenthe emitter of the transistor Q1 supplies the base of the transistor Q4with base current, due to which the transistor Q4 is conducting. In theconduction state, the transistor 04 pulls down the emitter voltage of Q2to the potential 0V almost without voltage loss. In consequence of this,the transistor Q3 having the base connected to the emitter of Q2 is off.Then there is no current through Q3. The multivibrator circuit has nowonly one current path, i.e. Rc1-Q1-C-Q4. The transistor Q3, whennon-conducting, separates one terminal of the capacitor C entirely fromthe potential 0V. The transistor Q4 being on connects the other terminalof the capacitor C to the potential 0V almost without voltage loss. Inthe other oscillating state, correspondingly, Q1 is off, Q2 is on, Q3 ison and Q4 is off. Then the multivibrator circuit has only one currentpath, i.e. Rc2-Q2-C-Q3. Q4 not being on separates one terminal of thecapacitor C entirely from the potential 0V. The transistor Q3 being onpulls down the other terminal of the capacitor C to the potential 0Valmost without voltage loss. It is thus possible to bring the greatestpossible part of the operating voltage across the capacitor. Due to thefact that voltage losses caused by current sources in conventionalmultivibrator circuits can be avoided thanks to the cross-connectedpull-down transistors Q3 and Q4, the double-cross-connectedmultivibrator circuit according to the invention implemented bypull-down technique produces an output signal amplitude twice as strongas the conventional circuits of FIGS. 1 and 2 at the same operatingvoltage.

The pulse shapes of the circuit of FIG. 3 are, however, relatively poor.This is due to the fact that even if the bipolar transistors Q3 and Q4are quick, the main problem with them consists in the high base currentsrequired. These disturb the recharge process of the reference capacitorC in an unacceptable way and increase the phase noise significantly.Also the speed of the multivibrator circuit is considerably lower thanin the conventional circuits of the FIGS. 1 and 2.

These problems relating to pulse shape, disturbances and speed can beeliminated by replacing the bipolar transistors Q1 to Q4 of FIG. 3 byMOS transistors, as shown in FIG. 4. The use of MOS transistorsdecreases the currents required for the control of the pull-downtransistors. The basic connection and operation of the circuit of FIG. 4are essentially similar to those of the circuit of FIG. 3. The circuitof FIG. 4 comprises four MOS transistors M1, M2, M3 and M4. A sourceelectrode of the transistor M1 is connected via a resistor Rc1 to apositive operating voltage Vcc of an operating voltage source 1 and adrain is connected to a source electrode of the transistor M3. A drainof the transistor M3 is connected to a lower operating voltagepotential, e.g. 0V, of the voltage source 1. A source electrode of thetransistor M2 is connected via a resistor Rc2 to the operating voltageVcc and a drain to a source electrode of the transistor Q4. A drain ofthe transistor M4 is connected to the operating voltage 0V. A capacitorC is connected between the emitters of the transistors M1 and M2. Apositive feedback is provided between the transistors M1 and M2 as wellas between the transistors M3 and M4 on the same principle as in FIG. 3.The operation of the circuit of FIG. 4 is also otherwise essentiallysimilar to the operation of the circuit of FIG. 3. An essentialdifference is, however, that a higher supply voltage, e.g. Vcc=3 V, hasto be used on account of the properties of the MOS transistors (highvolume resistances and low amplification, primarily). However, the pulseshape achieved is still poor and the frequency relatively low.

An improved multivibrator circuit based exclusively on MOS transistorsis shown in FIG. 5. The circuit of FIG. 5 is provided with MOStransistors M5 and M6 acting as buffer transistors, via which signalsfrom drain electrodes of transistors M1 and M2 are applied to control apull-down of pull-down transistors M3 and M4, respectively. To be moreexact, a source of the first buffer transistor M5 is connected to anoperating voltage Vcc and a drain electrode is connected to a gate ofthe pull-down transistor M4 and via a resistor R3 to an operatingvoltage potential 0V. A gate of M5 is connected to the drain electrodeof M1. Correspondingly, a source of the other buffer transistor M6 isconnected to the operating voltage Vcc and a drain electrode isconnected to a gate of the pull-down transistor M3 and via a resistor R4to the operating voltage potential 0V. The buffer transistors M5 and M6are capable of generating a higher control current for the gateelectrodes of the transistors M3 and M4, which current dischargesparasitic capacitances of the gate electrodes faster. This acceleratesthe switching of the transistors M3 and M4. Additionally, the control ofthe transistors M3 and M4 is separated from the proper resonancecircuit, which decreases disturbances in the operation of the resonancecircuit. A better pulse shape and a speed increase of about 15% areachieved by the multivibrator circuit of FIG. 5 compared to themultivibrator circuit of FIG. 4, but no lower operating voltage.

FIG. 6 shows a multivibrator according to the invention, implemented byBiCMOS technique. The circuit of FIG. 6 resembles the circuit of FIG. 3,but the bipolar transistors Q3 and Q4 are replaced by MOS transistors M3and M4 with their gate electrodes cross-connected to base electrodes ofthe transistors Q1 and Q2. This is possible, because the load caused bythe gate electrodes of the MOS transistors M3 and M4 is insignificantcompared to base currents of the transistors Q1 and Q2 and doestherefore not disturb the operation of the transistors Q1 and Q2. To beprecise, a collector electrode of the transistor Q1 in the circuit ofFIG. 6 is connected via a resistor Rc1 to a positive operating voltageVcc of an operating voltage source 1 and an emitter is connected to asource electrode of the transistor M3. A drain electrode of thetransistor M3 is connected to the operating voltage potential 0V. Acollector of the transistor Q2 is connected via a resistor Rc2 to theoperating voltage Vcc and an emitter to a source electrode of thetransistor M4. An emitter of the transistor M4 is connected to theoperating voltage 0V. A capacitor C is connected between the emitters ofthe transistors Q1 and Q2. A positive feedback is provided between thetransistors Q1 and Q2 in the same way as in FIG. 3. The gate electrodeof the transistor M3 is connected to the base electrode of thetransistor Q2 and the gate electrode of the transistor M4 is connectedto the base electrode of the transistor Q1. In consequence of thiscross-connection, Q2 and M3 are off when Q1 and M4 are conducting, andQ1 and M4 are off when Q2 and M3 are conducting. Because the transistorsQ1 and Q2 in the circuit of FIG. 6 are bipolar transistors, the problemwith the circuit of FIG. 4, i.e. high operating voltage, is avoided.Thanks to the active pull-down operation implemented by the MOStransistors M3 and M4, the speed of the circuit is, however, the same asthat of the conventional circuits (FIGS. 1 and 2), but the amplitude istwice as high. It is therefore possible to use the circuit of FIG. 6 ata much lower operating voltage, e.g. 1.1V, than that of a conventionalmultivibrator circuit, where the same performance requires an operatingvoltage of 1.8V.

A further multivibrator circuit according to the invention is providedby applying the buffer transistor solution used in the circuit of FIG. 5to the circuit of FIG. 3. Such a multivibrator circuit is shown in FIG.7. The circuit comprises six NPN bipolar transistors Q1, Q2, Q3, Q4, Q5and Q6. A collector electrode of the transistor Q1 is connected via aresistor Rc1 to an operating voltage Vcc and an emitter is connected toa collector of the transistor Q3. A collector of the transistor Q2 isconnected via a resistor Rc2 to the operating voltage Vcc and an emitterto a collector of the transistor Q4. Emitters of the transistors Q3 andQ4 are connected to an operating voltage 0V. A capacitor C is connectedbetween the transistors Q1 and Q2. A positive feedback is providedbetween the transistors Q1 and Q2 in the same way as in the circuit ofFIG. 3. Additionally, the circuit of FIG. 7 is provided with buffertransistors Q5 and Q6, through which signals are applied from bases ofthe transistors Q1 and Q2 to base electrodes of the transistors Q3 andQ4. Thanks to the buffer transistors Q5 and Q6, higher base currents canbe generated for the transistors Q3 and Q4, which accelerates thedischarge of parasitic capacitances of the base electrodes and thus theswitching speed of the transistors Q3, Q4. To be precise, a collectorelectrode of the buffer transistor Q5 is connected to the operatingvoltage Vcc and an emitter is connected to a base electrode of thepull-down transistor Q4 and via a current source 71 to the operatingvoltage potential 0V. A base electrode of Q5 is connected to a baseelectrode of Q1. A collector of the buffer transistor Q6 is connected tothe operating voltage Vcc and an emitter is connected to the baseelectrode of the pull-down transistor Q3 and via a current source 72 tothe operating voltage potential 0V. A base electrode of Q6 is connectedto a base electrode of Q2. The emitters of the transistors Q3 and Q4 areinterconnected and connected via a current source 73 to the operatingvoltage potential 0V. By means of the current sources 71, 72 and 73, theoperating point of the circuit can be set and the resonance frequencycan be determined. The circuit of FIG. 7 achieves an about 40% higherspeed than any other of the circuits described above. The pulse shape isalso one of the best among the alternatives described.

The circuit of FIG. 7 has been analyzed by using 0.8 μm BiCMOStechnology, in which bipolar NPN transistors have a transient frequencyF_(T) =14 GHz. The current flowing through the transistors is selectedin such a way that it provides this transient frequency F_(TMAX), thecurrent being about 850 μA on this technology while a collector-emittervoltage V_(ce) is about 0.8 V. In FIG. 7, the current source 71determining the current flowing through the transistor Q5 is therefore850 μA. Correspondingly, the current source 72 determining the currentflowing through the transistor Q6 is 850 μA. The current source 73determining the currents flowing through the transistors Q1, Q3 and thetransistors Q2, Q4, respectively, is 1.7 nA, when the oscillation isdesired to take place at the maximum frequency, in this case being about1.4 GHz. Further, collector resistors having the value of 150 ohm areused for achieving an amplitude of at least 400 mV for the differentialsignals of the collectors. The power consumption is about 5.0 mW fromthe operating voltage 2.2 V. The reference capacitor C is provided withthe value 1.0 pF and is for this technology a reasonably small capacitorof RF type.

The drawings and the related description are only intended to illustratethe invention. The details of the invention may vary within the scopeand spirit of the attached claims.

We claim:
 1. Multivibrator circuit, comprising:an operating voltagesource, a first bipolar transistor comprising a first and a second mainelectrode, and a base electrode, a second bipolar transistor comprisinga first and a second main electrode, and a base electrode, the firstmain electrode of the second bipolar transistor being connected tocontrol the base electrode of the first bipolar transistor, andrespectively, the first main electrode of the first bipolar transistorbeing connected to control the base electrode of the second bipolartransistor, a capacitive component connected between the second mainelectrode of the first bipolar transistor and the second main electrodeof the second bipolar transistor, a first and a second resistor, viawhich the first main electrode of the first bipolar transistor and thefirst main electrode of the second bipolar transistor, respectively, areconnected to a first potential of the operating voltage source, a firstpull-down MOS transistor connected in series between the second mainelectrode of the first bipolar transistor and a second potential of theoperating voltage source, and having a gate electrode, a secondpull-down MOS transistor connected in series between the second mainelectrode of the second bipolar transistor and the second potential ofthe operating voltage source, and having a gate electrode, the gateelectrodes of the first and second pull-down MOS transistors beingcross-connected to the base electrode of the second and the firstbipolar transistors, respectively.
 2. Multivibrator circuit,comprisingan operating voltage source, a first amplifier componentcomprising a first and a second main electrode and a control electrode,a second amplifier component comprising a first and a second mainelectrode and a control electrode, the first main electrode of thesecond amplifier component being connected to control the controlelectrode of the first amplifier component, and respectively, the firstmain electrode of the first amplifier component being connected tocontrol the control electrode of the second amplifier component, acapacitive component connected between the second main electrode of thefirst amplifier component and the second main electrode of the secondamplifier component, a first and a second resistor, via which the firstmain electrode of the first amplifier component and the first mainelectrode of the second amplifier component, respectively, are connectedto a first potential of the operating voltage source, a third amplifiercomponent connected in series between the second main electrode of thefirst amplifier component and a second potential of the operatingvoltage source, a fourth amplifier component connected in series betweenthe second main electrode of the second amplifier component and thesecond potential of the operating voltage source, a fifth amplifiercomponent, in which a first main electrode is connected to the firstpotential of the operating voltage source, a second main electrode isoperationally connected to the second potential of the operating voltagesource via a resistor or a current source, and a control electrode isconnected to the second main electrode or the control electrode of thefirst amplifier component, a sixth amplifier component, in which a firstmain electrode is connected to the first potential of the operatingvoltage source, a second main electrode is operationally connected tothe second potential of the operating voltage source via a resistor or acurrent source and a control electrode is connected to the second mainelectrode or the control electrode of the second amplifier component,the control electrodes of the third and the fourth amplifier componentsbeing cross-connected to the second main electrode of the fifth and thesixth amplifier components, respectively.
 3. Multivibrator circuitaccording to claim 2, wherein the first, the second, the third, thefourth, the fifth and the sixth amplifier components are bipolartransistors and that the control electrode of the fifth and the sixthamplifier components is a base connected to the control electrode of thefirst and the second amplifier components, respectively, the controlelectrode of the first and second amplifier components being a base. 4.Multivibrator circuit according to claim 2, wherein the first, thesecond, the third, the fourth, the fifth and the sixth amplifiercomponents are MOS transistors and that the control electrode of thefifth and the sixth amplifier components is a gate connected to thesecond main electrode of the first and the second amplifier components,respectively.
 5. Multivibrator according to claim 3, wherein the secondmain electrodes of the third and the fourth amplifier components areemitters, which are interconnected and connected via a first currentsource to the second potential of the voltage source.
 6. Multivibratorcircuit according to claim 5, wherein the second main electrode of thefifth and sixth amplifier components, this electrode being an emitter,is connected via a second and a third current source, respectively, tothe second potential of the voltage source.